The present invention relates to an optoelectronic semiconductor device, and in particular, to an optoelectronic semiconductor device having heterojunctions.
In an ordinary semiconductor laser, on an active layer formed in a substrate thereof, laminated layers are epitaxially grown to form heterojunctions between the active layer and the laminated layers. In the laser, a light produced therein is propagated through the active layer in a planar direction thereof, thereby emitting the light from a side surface thereof. Although a plurality of lasers may be integrally manufactured in a single chip, the arrangement of the lasers with respect to lights radiated therefrom is limited due to the configuration thereof to, for example, a one-dimensional disposition on the same plane.
In a semiconductor laser of a plane or surface emission type (to be referred to as a surface emission laser herebelow), a laser beam is emitted in a direction perpendicular to a substrate surface thereof. In a case where a plurality of surface emission lasers are manufactured as an integrated circuit in a single chip, laser lights emitted therefrom can be arbitrarily arranged in a two-dimensional plane.
Consequently, the surface emission laser is quite advantageously applicable to light communications such as a multiplexed light communication employing laser beams. In the following paragraphs, description will be given of the present invention primarily, but not limitatively, in conjunction with the surface emission laser by way of example.
In a surface emission laser, to form an optical cavity, a reflection mirror is disposed on each of the opposing end sides of an active layer conducting light. The mirror can be configured in the form of a distributed Bragg reflector (DBR) including a multi-layered film or stack developing a periodic change in a refractive index thereof. When the change occurs at an interval of an optical length .lambda./4 or a quarter wavelength, the sum of multiple reflection of light having a wavelength .lambda. takes a maximum value thereof. FIGS. 6A and 6B show states of a main section of a surface emission laser according to the conventional technique. FIG. 6A schematically shows a band structure of an active layer, clad layers disposed on both sides thereof, and DBR layers formed on both end sides of the resultant constituent block including the active layer and the clad layers. In the band structure diagram, the ordinate represents values of energy and the abscissa stands for positions along depth of the laser.
In the diagram, an active layer 51 has a predetermined energy gap Eg. Disposed on both ends of the active layer 51 are clad layers 52A and 52B each having a band gap larger than that of the active layer 51 and a refractive index smaller than that of the active layer 51. Each of the active layer 51 and clad layers 52A, and 52B is configured with a non-doped semiconductor layer. Each of the clad layer may be replaced with a laminated of a plurality of different layers.
On an outside end of the clad layer 52A, there are laminated semiconductor layers forming a first DBR structure 53A. Moreover, on an outside end of the clad layer 52B, semiconductor layers are similarly piled to form a second DBR structure 53B.
The first DBR stack 53A includes a semiconductor layer 54A having a wide band gap and a semiconductor layer 55A having a narrow band gap, which are successively arranged in an alternate manner. The second DBR stack 53B includes a semiconductor layer 54B of a wide band gap and a semiconductor layer 55B of a narrow band gap, which are similarly disposed in an alternate manner.
Each of the layers 54A, 54B, 55A, and 55B has an optical length .lambda./4 for a wavelength .lambda. of light radiated from the active layer 51. In this connection, the optical length .lambda./4 implicitly includes 3.lambda./4, 5.lambda./4, etc. developing optically the same function as .lambda./4. Furthermore, the optical length need not be exactly identical to .lambda./4. Particularly, the layer thickness of the outer-most layer has a wide allowance for the optical length.
In a semiconductor, a refractive index is related to a band gap thereof. Generally, a semiconductor having a wide band gap possesses a small value of refractive index. Consequently, each of the DBR structures 53A and 53B develops a variation in a refractive index according to a change in the band gap thereof, and a hetero-boundary surface thereof forms an optical boundary surface. Since each semiconductor layer has the thickness .lambda./4, the light emitted from the active layer 51 and having the wavelength .lambda. is reflected by the DBR structures 53A and 53B. Namely, this configuration forms a resonator.
In order to cause radiative recombinations in the active layer 51, it is only necessary to dope impurity substances in the DBR stacks 53A and 53B so as to constitute a pin diode structure.
FIG. 6B shows a band structure developed when a pin structure obtained by doping a p-type impurity substance to the DBR structure 53A and an n-type impurity substance to the DBR structure 53B is under the thermal equilibrium.
In a semiconductor layer having a wide band gap, since carriers including electrons and holes are moved to a semiconductor layer having a narrow band gap, there appears space (fixed) charges of a reverse polarity. This leads to a band bending effect, namely, the band is curved.
The curve impedes the carrier transfer through a semiconductor layer having a wide band gap. Since there are formed a plurality of such potential barriers, the forward-directional resistance of the pin diode becomes to be increased, for example, to several kiloohms (k.OMEGA.).
FIGS. 7A and 7B illustratively show states of a pin diode configuration including DBR structures capable of decreasing the forward resistance. In FIG. 7A, the diode is in the abscence of an electric field; whereas, in FIG. 7B, the diode is under the thermal equilibrium.
In FIGS. 7A and 7B, there appears a composition gradient in the neighborhood of each of the hetero-boundaries of the DBR stacks 53A and 53B and hence the band gaps thereof are gradually changed. The spike-like potential barrier does not appear due to the composition gradient and the doping. This prevents an abrupt potential barrier from being formed therein. There has been reported that the resistance of a pin diode is decreased to about 12 ohms by use of the device configuration as above realized by equivalent superlattice structure.
Incidentally, when manufacturing a surface emission laser, it is essential to exactly control arrangement of each layer constituting the DBR structures.
When each of the DBR layers is manufactured to have a uniform composition, it is difficult to lower the resistance of the pin diode. Although the resistance can be decreased by forming heterojunctions of the composition gradient type as above, the regions of the composition gradient are required to be fabricated in a region of the order of 100 angstrom (.ANG.) with each layer of a thickness of 20 to 100 angstroms (.ANG.), which cannot be easily achieved. Consequently, it is quite difficult to manufacture, according to the prior art, a surface emission laser which includes the DBR structures of the favorable configuration as above and which hence develops a low forward resistance.